1. Field of the Invention
The present invention relates to a plasma torch for transmitted arcs with a central electrode and a concentric nozzle endpiece. Between the electrode and the nozzle endpiece, there is an annular gap which can be supplied with plasma gas by means of an annular passage. Also, a concentric torch casing is provided, having an outer wall, a middle wall and an inner wall, whereby between the nozzle endpiece and the torch casing, on the end, there is an annular passage, the inner wall of which annular passage is partly formed by a tubular insulation arrangement electrically isolating the two parts. This annular passage can be flushed with plasma gas to clean any contaminants therefrom.
2. Background Information
One essential problem in the operation of plasma torches, in particular when alternating current and three-phase current are used, is the occurrence of parasite arcs which burn, or arc, parallel to the principal arc. In particular, the parasite arcs include the bottom edge of the lower nozzle or torch casing and the outer area of the nozzle or torch end in the current flow. Not only do parasite arcs have an adverse effect on the stability of the arc column, and thus the efficiency and the economy of a plasma torch, or of a system operated with plasma torches, but these parasitic arcs can even lead to the complete destruction of plasma torches.
To prevent parasite arcs, Federal Republic of Germany Patent No. 33 28 777 discloses the addition of an electrically insulating lining in the annular passage between the electrode and the nozzle on the nozzle inside. Having an electrically insulating lining in the annular passage provides only partial protection, since parasite arcs can find a current path outside the insulating lining.
An additional measure to combat parasite arcs is disclosed in Federal Republic of Germany Patent No. 34 35 680. Particularly, as disclosed in this patent, the section of the inside wall of a water-cooled nozzle adjacent to the end wall is electrically insulated from the segment of the end wall adjacent to the outer wall part by two separate insulating parts which penetrate the wall segment in question via the entire cross section surface. However, for very hot furnace atmospheres, it is very difficult to find a suitable insulation material for the end wall of the nozzle.
In a refinement of the theory described immediately above, the insulation provided in the end surface of the nozzle is laid in a groove on the end surface. A plasma torch employing such a refinement has also been realized. The groove on the end side is essentially formed on one side by the outside wall of a nozzle tube or endpiece, and on the other side by a torch casing. The end of the torch casing has a flange pointing toward the axis of the torch to offer a certain degree of thermal protection to the recessed insulation piece. However, if this type of torch is operated in an atmosphere which contains electrically conducting particles, e.g. metal or metallurgical dusts, then the electrically conducting dusts can be deposited on the cooled insulation piece. In such an instance, an electrical bridge can be formed from the nozzle tube to the torch casing, and parasite arcs can flow via the outer edge on the end of the torch casing.